Certain clinical and mitral valvular morphologic findings are described in 83 patients (aged 26-79 years (mean 60); 26 women (31%) and 57 men (69%) with mitral valve prolapse (MVR) and mitral regurgitation (MR) severe enough to warrant mitral valve replacement. In each valve a portion of posterior mitral leaflet was elongated such that the distance from distal margin to basal attachment of this leaflet was similar to the distance from distal margin of anterior leaflet to its basal attachment to left atrial wall. Two major mechanisms for the severe MR were found: 1) dilatation of the mitral anulus with or without rupture of chordae tendineae, and 2) rupture of chordae tendineae with or without dilatation of the mitral anulus.